Method for scanning sequence selection for displays

ABSTRACT

The present invention refers to a method for scanning sequence selection for displays. In one scanning sequence selection for displays having a plurality of rows and columns, the plurality of rows and columns cross each other defining a plurality of optical elements having a first optical state and a second optical state in response to a first electric state and to a second electric state. The method includes the phases of driving the plurality of rows of the display according to a prefixed scanning ordering. The prefixed scanning ordering is predisposed by ordering every column of the columns so that the total switching number between said first electric state and said second electric state is minimized.

FIELD OF THE INVENTION

[0001] The present invention refers to a method for scanning sequenceselection for displays.

BACKGROUND OF THE INVENTION

[0002] An LCD display (Liquid Crystal Display) is generally formed by amatrix of row and column electrodes that, opportunely driven through theapplication of a voltage signal, define at the intersection points, theso-called pixels, a change of the optical behaviour of the interposedliquid crystal.

[0003] The driving of an LCD display through traditional techniquerequires that in correspondence of a unique selection pulse, applied toa certain row, the voltage values able to determine the turning on orthe turning off of all the pixels of the excited row are applied on thecolumn electrodes. The image on the display is completed in the momentin which the last row electrode has been selected. It is indicated withthe word “frame” the lapse of time within which the complete scanning ofthe row electrodes multiplexed according to the just describedprinciple, is completed. Such method is known as Alt & Pleshko (A&P),or, in its improved version, as Improved Alt & Pleshko (IA&P).

[0004] It is known nevertheless that the IA&P method sets some problemsin the case of display with a great number of rows, because of thephenomenon called “frame response” and because of the great supplyvoltage required. To obviate to such drawbacks a technique calledMulti-Line-Addressing (MLA) has been introduced in which a plurality ofrow electrodes is simultaneously selected through opportune orthonormalwaveforms. Such a technique has been used in a great number ofvariations, that differ, for example, in the different temporaldistribution of the excitement pulses or for the different grouping ofthe rows simultaneously selected.

SUMMARY OF THE INVENTION

[0005] The Applicants have realized that all these variations arenevertheless joined by having a prefixed scanning ordering which isindependent from the specific image that is intended to visualize.Besides, the current consumption of the driving devices is a function ofthe current required by the panel, which depends on the informationpattern that is intended to visualize. At the varying of the informationpattern, in fact, the charge that must be transferred to the panelvaries, both for the switchings on the row electrodes (row) and for theswitchings on the column electrodes (column). Therefore a consumptiondependence of the panel, that visualizes a well defined informationpattern, on the waveforms applied to the rows and on the waveformsapplied to the columns exists.

[0006] In view of the state of the art described, an object of thepresent invention is to provide a method for the selection of thescanning sequence for displays, which has reduced consumes.

[0007] In accordance with the present invention, such object is reachedby means of a method for scanning sequence selection for displays havinga plurality of rows and columns, wherein said plurality of rows andcolumns cross each other defining a plurality of optical elements havinga first optical state and a second optical state in response to a firstelectric state and to a second electric state, the method comprising thephases of driving said plurality of rows of said display according to aprefixed scanning ordering; characterized in that said prefixed scanningordering is predisposed by ordering every column of said plurality ofcolumns so that the total switching number between said first electricstate and said second electric state is minimised.

[0008] Preferably, said prefixed scanning ordering is predisposed byordering every column of said plurality of columns in such a way that,if the state change between the row i and the row j is different fromthe state change between the row i and the row i+1, then the scanningchange is effected between the row i+1 and the row j. Advantageously,before effecting said ordering a further ordering is effected for everycolumn of said plurality of columns by grouping the rows having thegreatest number of said first electric state.

[0009] More advantageously, before effecting said ordering a furtherordering is affected for every column of said plurality of columns bygrouping the rows for number of presences of said first electric state.

[0010] Thanks to the present invention it is possible to realize amethod for the selection of the scanning sequence for displays which isable to minimize the current consumptions, further to some undesiredoptical effects determined by the switchings of the driving waveforms,through the reduction of the switching number of the column signals,obtained through the determination of an optimal scanning ordering,built on the image that is intended to visualize.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The features and the advantages of the present invention will bemade more evident by the following detailed description of a particularembodiment, illustrated as a non-limiting example in the annexeddrawings, wherein:

[0012]FIG. 1 shows the time varying behaviour of the row and columnsignals in accordance to a first display driving method;

[0013]FIG. 2 shows the time varying behaviour of the row and columnsignals in accordance to a second display driving method;

[0014]FIG. 3 shows a flow chart relative to an ordering method for thescanning succession of the rows of a LCD display driven with theAlt&Pleshko technique, in accordance to the present invention;

[0015]FIG. 4 shows an image visualized by a display;

[0016]FIG. 5 shows a flow chart relative to a succession ordering methodof the row scanning of a LCD display driven with the MLA technique, inaccordance to the present invention;

[0017]FIG. 6 shows a block diagram of a circuit for the display controlin accordance to the known art;

[0018]FIG. 7 shows a block diagram of a circuit for the display controlin accordance to a first embodiment of the present invention;

[0019]FIG. 8 shows a block diagram of circuit for the display control inaccordance with a second embodiment of the present invention.

DETAILED DESCRIPTION

[0020] We now consider the driving technique known as Alt&Pleshko,together with its variations, that are characterized by the excitementof only one row of the display in every elementary instant of time.Starting from the simplest situation of a black and white display, it isobserved that in the passage from the row N to the row N+1 the columnwaveforms can be subjected in general to assume a different voltagevalue. This happens, more precisely, if given the couple of pixel (Ri,Cj) and (Ri+1, Cj) where Ri identifies the row i and Cj identifies thecolumn j, the state of such pixel is different. Under these conditionsthe voltage waveform of the column j will have a switching that allowsconsidering such passage of state. Such switching determines a currentconsumption at electric level and a reduction of contrast at opticallevel, through a multiplicity of mechanisms.

[0021] As it can be seen in FIG. 1, in order to visualize the imagehaving the numerical reference 10 on a display having 4 rows and 3columns, the rows R0-R3 must have a driving signal as reported in figurewith the numerical reference 11, and the columns C0-R2 must have adriving signal as indicated in the figure with the numerical reference12. In this first case there are four switchings of the column signalfor every row, that is 12 switchings.

[0022] Instead, as it can be seen in FIG. 2, in order to visualize thesame image having the numerical reference 10, the driving signal of therows R0-R3 has been modified and reported in the figure with thenumerical reference 13, and the driving signal of the columns C0-R2 hasbeen modified accordingly and indicated in the figure with the numericalreference 14. In this second case there will be only two switchings ofthe column signal for every row, that is 6 switchings.

[0023] Supposing to have a display constituted by a single column, itcould be possible to think about minimizing its switching number throughthe scanning of all the rows whose pixel is in the same state, followedby the scanning of all the rows in which the pixel is in the oppositestate, whichever are the initial and final states, with an onlyintermediary switching, astride the two pixel groups. Since there is aplurality of rows in the display, at the varying of the image to bevisualized, of course, it is not possible, in general, to reduce to 1the switching number of all the columns; nevertheless it is possible toget the minimum number of switchings through an opportune ordering ofthe rows which considers the differences among the pixels of a given rowand any other.

[0024] The method reported in FIG. 3 is proposed, that, given the numberof total rows, requires n*(n−1)*(½) steps for the determination of theoptimal ordering. The method produces as a result a vector, whoseelements i are the indexes of the rows that it is necessary to excite insequence to minimize the consumptions.

[0025] Referring now to FIG. 3, the ordering method of the scanningsuccession of the rows of a LCD display driven with the Alt&Pleshkotechnique will be described, in accordance to the present invention.

[0026] At the step 30 a first cycle opens, with variable i, that checksall the rows from the row 1 to the row n−2.

[0027] At the step 31 a second cycle opens, with variable j, that checksall the rows from the row 1+2 to the row n.

[0028] At the step 32 a check is effected to verify if the state changebetween the row i and the row j is different with respect to the changebetween the row i and the row i+1. In affirmative case T we proceed atthe step 33, in negative case F we proceed to the step 34.

[0029] At the step 33 the exchange between the row i+1 and the row j iseffected.

[0030] At the step 34 the variable j is increased and we proceed to thestep 31. When the variable j has reached the value n we proceed with thestep 35.

[0031] At the step 35 the variable i is increased and we proceed to thestep 30.

[0032] The method described in accordance to a generic programminglanguage is:

[0033] FOR i=1 TO n−2

[0034] FOR j=i+2 TO n

[0035] If Changes (i,j) <> Changes (i, i+1) THEN swaps (i+1, j)

[0036] NEXT j

[0037] NEXT i where “Changes (i,j)” means the state change between therow i and the row j, and “swaps (i+1,j)” means the exchange between therow i+1 and the row j.

[0038] In the case of the image proposed in FIG. 4, for example, wechange from a number of switchings equal to 6266, in the scanning forcontinuous rows, to a number of switchings equal to 422, with theoptimal sequence, in accordance to the present invention.

[0039] In the case of more columns the operation of Changes (i,j) mustbe intended as state variation between the whole row i and the whole rowj, or rather as calculation of the changes on all the columns of the tworows. In other terms a comparison between the two rows is globallyeffected.

[0040] In the case of MLA technique, the problem is set in more complexterms for the fact that before establishing what ordering the groups ofrows simultaneously excited must follow, it is necessary to establish asthese quatrains are composed, or what rows must be selected for beingexcited together. Particularly, a solution is provided for the so-called“Distribuited MLA”, in which the p pulses associated to a single row(having indicated with p the row number simultaneously selected) areequispaced in the time, as this is the solution that allows to bestsolve the problem of the frame response. With the MLA technique, thewaveform of column does not assume a certain value in function of thestate of a single pixel, but in function of the state of more pixels;particularly, the voltage value resulting for the waveform of the columnelectrode is determined by the calculation of the number of differentbits among the so-called “row pattern” (given by the state of the fourrow voltages applied to the four rows simultaneously selected) and theso-called “information pattern”, that is the vector of four pixels thatlie at the intersection between a given column and the four selectedrows.

[0041] A first immediate extension of the algorithm previously describedfor the A&P technique is possible by ordering in accordance to thepreceding method the row signals, and applying, in succession, the psimultaneous selection pulses to the p rows, that is to the p rowsdefined by adjoining vector indexes, for example, in the typical case ofusing 4 simultaneously driven rows (p=4), by exciting together the rowspointed by the positions from 1 to 4 of the vector previous ordered fordifferences, then proceeding with the rows from 5 to 8 and so on.

[0042] This way of proceeding has got a meaning because, if theobjective is that to have excited in succession on each column groups of4 pixels (belonging to the same row number) which are maximally similar,so that compared to the same row pattern the difference, and thereforethe voltage, is also maximally similar, then the ordering fordifferences before described tends really to make rows closer,considered also in groups of 4, besides to 2 by 2, which are still verysimilar, although the described method can be improved.

[0043] Nevertheless, it is possible to improve the method beforedescribed, by preceding the ordering for differences among adjoiningrows (or rather the method previously described) with an ordering forequal number of bits on each single row separately. The first part ofthe method consists in fact in ordering (in increasing or decreasingordering, indifferently) for number of present zeros (or ones).

[0044] In forming a scanning row quatrain we refer to a n-row belongingto the first quarter of this orderly set, to a n-row of the secondquarter and so on, at the end the information patterns will bedistributed in such a way that all of them have a number of 1/0 verysimilar for construction, even if it is not known where such 1/0 are.

[0045] The second part of the method consists in an ordering fordifferences (or state changes), as the one previously proposed, butwhich acts in an independent way in the four sections in which theordered list of the rows for equal bit number will be divided.

[0046] At this point, there are taken the first row of each of thep-sections (constituting the first group of p-rows simultaneouslyselected) in which the list has been divided, the second of eachp-section and so on, up to complete the first frame quarter (after N/pgroups of selected rows), then repeating other p−1 scanning up tocomplete the whole frame.

[0047] This new method is schematised in FIG. 5.

[0048] At the step 50 a first cycle opens scanning all the rows.

[0049] At the step 51 the number of the 0 present in each row iscounted.

[0050] At the step 52 the first cycle is closed.

[0051] At the step 53 the row vector is ordered (in increasing ordecreasing ordering, indifferently) for the number of present zeros. Theordering method can be anyone known to the technician of the field.

[0052] At the step 54 a second cycle opens scanning all the rows.

[0053] At the step 55 the method before described is applied, that isthe vector of the rows ordered at the step 53 is now ordered so as toreduce the switching number between a row and the other.

[0054] At the step 56 the second cycle is closed.

[0055] With the ordering for zero numbers we have that in each quatrain(in the case of p=4), altogether the number of 1/0 is maximally similar,and with the ordering for differences it is guaranteed that in everyinstant of time the selected row in the group i (1<i<p) has a minimumnumber of switchings in comparison to the preceding one (precedinginstant) of the same group.

[0056] The computational cost that involves the ordering for zerosnumbers is function of the type of method of ordering used; it can varyfrom the n(n−1)/2 steps for the ordering method defined “bubblesort” tothe n*log2(n) steps of the ordering method defined “quicksort”. Theordering for differences instead involves a computational complexitywith a number of steps equal to (n/p)*(n/p−1)*(p/2).

[0057] In function of the row number of the display and the refreshmentfrequency of the images, in order to effect the comparisons among therows it can be enough to lean to the normal mechanism for reading thememory RAM during the visualization of its content or an independentreading and comparison mechanism can be predisposed that accelerates theordering process and therefore increases the current saving through therapid individualization of the minimum consumption configuration.

[0058] We refer now to FIG. 6 that shows a block scheme of a controlcircuit of a display in accordance to the known art.

[0059] In this figure the architecture of a typical LCD controller isreassumed, valid both in the case of driving in accordance to the A&Ptechnique, and in the case of driving in accordance to the MLAtechnique, from the moment that the A&P technique can be considered as adegenerate case of the MLA technique, obtained with a row numbersimultaneously driven equal to 1.

[0060] It includes a display 60 that receives the driving signals from arow driving stage 61 and a column driving stage 65.

[0061] A RAM memory 64, whose output is applied to a correlator 63, isused for the storing (buffer memory) of the information to bevisualized, and it allows the reduction of the data flow that themicrocontrollor (not shown) sends to the LCD controller throughmaintenance of a local copy.

[0062] A row configuration generator 62 defines the row waveforms,proposing it in coherent form both to the row driving stage 61 and tothe correlator 63 through which the column waveforms will be calculated.

[0063] The correlator 63, that receives data from the row configurationgenerator 62 and from the RAM 64, furnishes data to the column drivingstage 65. The correlator 63 allows to value, during the p refreshmentscannings of the LCD display associated to the presence of p rowssimultaneously selected, the appropriated column voltage value. Ingeometric terms, this block values the orthogonal projection of thecolumn waveform on one of the p row functions that constitute a completebase of the space of the column waveforms. The typical realization ofthis block at circuital level foresees, for a generic column Cj, thecalculation of the bit-to-bit differences among p bits of information(extracted by the RAM 64 on the cells of the column j on the p rowsselected in a certain instant) and the p bits descriptive of the rowwaveform applied in the same instant to the p selected rows. In thedegenerate case of driving in A&P, the complete orthonormal base of therow waveforms is constituted by identically null functions except thatin an only instant, correspondent to that of activation of the selectedrow.

[0064] The row driving stage 61 receives data from the row configurationgenerator 62 and furnishes its output data to the display 60. The rowdriving stage 61 effects the association among the descriptive binarycode of the row configuration in a certain instant and the correspondingvoltages, besides the conditioning of the necessary signal to guaranteean output impedance sufficiently reduced, so as to limit the deformationof the waveform applied to the display.

[0065] The column driving stage 65 effects the association among thedescriptive binary code of the column configuration in a certain instantand the corresponding voltages, besides the conditioning of thenecessary signal to guarantee an output impedance sufficiently reduced,so as to limit the deformation of the waveform applied to the display.

[0066] A RAM address generator 66 produces the access addresses in RAM64 in progressive reading, or a simple counter to fix the ordering ofaccess to the RAM 64.

[0067] Referring now to FIG. 7 that shows a block scheme of a displaycontrol circuit in accordance to a first embodiment of the presentinvention. The blocks similar to those of FIG. 6 have the same numericalreference.

[0068] In the method previously described takes place, in every instant,the evaluation of the number of bits that change among a reference row“i” (already placed in the correct ordering in comparison to thepreceding ones) and the following “i+1” (that is the more closest to therow “i” found up to now) and the evaluation of the bit number thatchange among the same reference row “i” and the next “j” candidate to bethe row that follows “i”, for the fact to be very similar.

[0069] Therefore, in FIG. 7 we find a generation block 73 of theevaluation addresses that has really the purpose to generate all thepairs of necessary rows to give a complete evaluation of the scanningordering which is in absolute the best for a globally minimum number ofdifferences among all the articulated rows. If we suppose that theordering occurs “off- line”, or if among the end of a scanning frame ofthe display 60 and the beginning of the following one the necessary timeto complete the ordering will be available, this block can complete theaforesaid ordering, through the access in RAM 64, dictated not by thedemand to bring the information to the display 60, but by the demand tocomplete the ordering.

[0070] In FIG. 7 we find a row adapter counter 70, that receives datafrom the RAM 64 and furnishes data to the correlator 63 and to aregister logic 72. The row adapter counter 70 consists of a couple ofregisters, able to determine the number of differences among the row ifirmly memorised in a register and the row j saved in the otherregister. At each following comparison between i and j, if theadaptation degree is greater than the excellent one previously found,then in the register logic 72 the pointer is adjourned to the next row(i+1), in such a way that it points to the previous row in the positionj. In the register logic 72 the pointer is updated to the row j, nowpromoted to i+1, saving the previously row in the position i+1 (that isa swap is made). It is saved in the register logic 72 the new minimumdifference among the row i and the i+1(ex j), that will constitute thenew reference for the following comparisons.

[0071] The register logic 72 is therefore the block that allows “toannotate” at the algorithm progress the new ordering of the rows and theset variable necessary to the completion of the same method, or thestate of the ordering vector of the rows. The register logic 72 receivesdata from the row adapter counter 70 and furnishes data to the rowconfiguration generator 62.

[0072] At the completion of the ordering phase, it will be the sameregister logic 72 to determine the real scanning ordering of the display60 by making reference to the complete version of the row orderingvector.

[0073] We refer now to FIG. 8 that shows a block scheme of a displaycontrol circuit in accordance to a second embodiment of the presentinvention. The blocks similar to those of FIG. 7 have the same numericalreference.

[0074] In FIG. 8 we find a row counter 80 having value 0, that receivesthe data from the RAM 64 and furnishes data to the row adapter counter70 and to a preliminary organizer 81, which furnishes data to theregister logic 72. These two blocks allow to obtain the ordering fornumbers of zeros.

[0075] Having described and illustrated the principle of the inventionin a preferred embodiment thereof, it is appreciated by those havingskill in the art that the invention can be modified in arrangement anddetail without departing from such principles. I therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

1. A method of scanning sequence selection for displays having aplurality of rows and columns, wherein said plurality of rows andcolumns cross each other defining a plurality of optical elements havinga first optical state and a second optical state in response to a firstelectric state and to a second electric state, the method comprising:driving said plurality of rows of said display according to a prefixedscanning ordering; and ordering every column of said plurality ofcolumns so that the total switching number between said first electricstate and said second electric state is minimized.
 2. The methodaccording to claim 1 wherein said prefixed scanning ordering comprisesordering every column of said plurality of columns so that, if the statechange between row “i” and row “j” is different from the state changebetween row “i” and row “i+1”, then the scanning change is effectedbetween row “i+1” and row “j”.
 3. The method according to claim 1further comprising an ordering for every column of said plurality ofcolumns by grouping the rows having the greatest number of said firstelectric state.
 4. The method according to claim 1 further comprising anordering for every column of said plurality of columns by grouping therows having the greatest number of said second electric state.